Polyphosphazenes and the Process of Macromolecular Substitution

The properties of polymers depend on the structure of the backbone and the types of side groups linked to the backbone. Most well-known synthetic polymers have both the skeleton and the side groups derived from organic precursors and ultimately from petroleum. These have proved to be transformative in many areas of materials science and in other fields that were hitherto dominated by biological polymers, metals, or ceramics. However, many classical organic polymers have serious deficiencies such as flammability, biomedical incompatibility, sensitivity to high energy radiation, or unwanted persistence in the environment. This is partly a consequence of limits to the types of side groups that will survive or permit a normal polymerization process. Moreover, side groups introduced by classical polymer synthesis techniques generally cannot be exchanged for other groups after the polymer has been assembled. By contrast, one of the main access routes to poly(organophosphazenes) uses organic or inorganic nucleophiles to replace the chlorine atoms in poly(dichlorophosphazene), (NPCl₂)_n,in a process of macromolecular substitution. The resultant polymers have unique characteristics derived from both the inorganic skeleton and a wide range of different side groups. This provides hitherto inaccessible property combinations that are appropriate for medical devices, aerospace materials, fire-resistant textiles, batteries, and semiconductor components, many of which cannot be achieved through classical polymer science.